Carbon is an element that is very efficient in removing impurities. Even from the early ages, there is evidence that advanced societies were using carbon to purify their water.
It was then discoverd that by heating the carbon, the absorbtion properties increased significantly. This is because all the non-carbon molecule were released, leaving plenty of room for absorbtion.
Activated carbon is used in air purifiers to remove volatile organic compounds from the air. These chemicals have the ability to cause respitory problems for some people.